The present invention relates to synchronous motors, in more detail to synchronous motors which consist essentially of a permanent-magnet-containing rotor and a stator; which, at startup, uses rectifiers and a commutator comprising sliders and brushes both rotary in unison with the rotor to supply the stator with only the rectified component, effective to energize the rotor's rotation, of the AC power-supply; and which are provided with a mechanical conversion device which, when the rotation of the rotor has reached nearly the synchronous rotation, releases the contact load by pushing the slider in the direction of rotary shaft until it moves out of the contact with the brush and at the same time supplies the stator directly with the AC power-supply.
A device of this kind is described by Gianni A. Dotto et al., "SYNCHRONOUS MOTOR STARTER CIRCUIT EMPLOYING COMMUTATOR AND RECTIFIER DURING STARTING," U.S. Pat. No. 3,394,296, which consists essentially of a permanent-magnet-type inner-side rotor and an outer-side stator; which, at startup, use rectifiers and a commutator comprising sliders and brushes both rotary in unison with the rotor to supply the stator with only the rectified component, effective to energize the rotor's rotation, of the AC power-supply; and which, when the rotation of the rotor has reached the synchronous rotation, allows a centrifugal switch provided in the circuit to short the circuit so as to supply the stator directly with the AC power-supply.
However, with such a device that keeps the slider in contact with the brush even after synchronous operation, the contact load consumes a considerable amount of energy and the contact attrition results in considerable decrease in durability.
The first purpose of the present invention is to provide synchronous motors that are capable of automatic release of load resistance during synchronous rotation; the second purpose is to provide synchronous motors that contain a brush and a commutator with greatly augmented durability; the third purpose is to provide synchronous motors that may be produced with reduction in size, weight, and cost through placing the permanent-magnet rotor outside; and the fourth purpose is to provide synchronous motors which have a permanent-magnet rotor in correspondence with a stator, which comprise rectifiers for converting the power-supply AC into rectified current and a commutator for switching the direction of current as the rotor rotates, which possess a conversion circuit capable of supplying the stator coil, during startup, with current constantly in the direction toward energizing the rotation, and which are provided with a constant-speed thrust device designed in such a way that the part of commutator rotary in unison with the rotor is capable of pushing in the direction along the rotary shaft until it is released from the stationary brush section, and that, when the rotation of the rotor has reached the speed for driving as a synchronous motor, the part rotary in union with the rotor is allowed to become disengaged and at the same time the circuit becomes shorted so as to supply the stator directly with the AC power-supply.